The present invention relates to a transfer-fixing belt for an electronic appliance and a method of manufacturing the same, particularly, to a fixing belt for a fixing device used in a electrophotographic machine, particularly, a copying machine, a color printer or a facsimile, for melting a toner and fixing the molten toner to a paper sheet or the like and a method of manufacturing the same.
It was customary to use a transfer roll made of a sponge rubber having a semi-conductivity for transferring a toner from a photosensitive drum onto a paper sheet in an electronic appliance using an electrophotographic system such as a copying machine, a color printer or a facsimile. The transfer roll includes, for example, a semi-conductive sponge roll having a hardness of about 30xc2x0 (Asker C type), which is prepared by mixing a conductive powder with an ethylene-propylene rubber to control the electrical resistance at 108 to 109xcexa9xc2x7cm, as described in xe2x80x9cJapan Hardcopy ""91, p 27).
FIG. 7 shows the concept of a copying machine (or a color printer). As shown in the drawing, the copying machine includes a photosensitive drum 1. A transfer roll 2, a developing roll 3, and a charging roll 4 for charging the photosensitive drum 1 are arranged in the vicinity of the photosensitive drum 1. Also, a fixing device 7 consisting of a fixing roll 5 and a pressurizing roll 6 arranged to face the fixing roll 5 is arranged on the downstream side of the photosensitive drum 1. In the copying machine of the particular construction, a toner image is formed by the developing roll 3 on the photosensitive drum 1 and transferred by the transfer roll 2 onto a paper sheet 8. Then, the paper sheet 8 bearing the toner image is transferred through a clearance between the fixing roll 5 heated to about 200 to 250xc2x0 C. and the pressurizing roll 6. As a result, the toner image is fixed to the paper sheet 8. At the same time, the toner image is smoothed by the pressure applied to the paper sheet 8 when the paper sheet 8 passes through the clearance between the rolls 5 and 6.
FIG. 8 shows in a magnified fashion the fixing portion, which is called a roll-nip system, shown in FIG. 7. As shown in FIG. 8, the pressurizing roll 6 comprises a mandrel 9 made of aluminum and a surface layer 10 covering the mandrel 9. The surface layer 10 consists of a heat-resistant resin adapted for releasing the toner such the heat resistant resin sold under the trademark TEFLON(copyright). On the other hand, the fixing roll 5 comprises a mandrel 12 having a heater 11 housed therein, a lower layer 13 covering the mandrel 12 and formed of a heat-resistant soft silicone rubber (or silicone sponge), and a PFA tube (or FEP tube) 14 covering the lower layer 13 and serving to facilitate release of the toner.
An oil supply roll 15 for coating the fixing roll 5 with a silicone oil is arranged near the fixing roll 5. The silicone oil coating prevents a toner 16 from being attached to the fixing roll 5. Further, a cleaning device 17 for removing the residual toner and paper dust remaining on the fixing roll 5 is arranged near the fixing roll 5.
Several problems must be solved for increasing the copying speed in the roll-nip system described above. It should be noted that a predetermined contact time between the rolls is required for melting the toner and fixing the molten toner to the paper sheet. For ensuring a long contact time, it is conceivable to increase the thickness of the rubber layer or to employ a wide-nip system in which the diameter of the rubber roll is increased so as to increase the nip width (distance of the contact portion between the fixing roll 5 and the pressurizing roll 6). However, the apparatus is rendered bulky and the heat conduction is lowered by the increased rubber amount, making it substantially impossible to put these techniques to practical use. It is also proposed to use a softer solid silicone rubber so as to bring about bulge during the contact time and, thus, to increase the nip width. However, an appreciable effect cannot be obtained in this case.
Under the circumstances, new proposals are being made for increasing the copying speed by employing a wide-nip technique. Specifically, known is a belt-nip system using the fixing roll 5 and a transfer-fixing belt 18 in combination as shown in FIG. 9. As shown in the drawing, the transfer-fixing belt 18 is stretched about a pressurizing roll 19 in contact with the fixing roll 5 via the belt 18, a supporting roll 20 and a driving roll 21.
Each of Japanese Patent Disclosure (Kokai) No. 61-132972 and Japanese Patent Disclosure No. 5-150679 discloses a wide-nip technique using a fixing device of a belt-nip system in place of the conventional roll-nip system for increasing the nip width with the fixing roll and, thus, increasing the copying speed.
JP ""972 noted above proposes a method of forming a belt with a thin film for shortening the warm up time. In this prior art, a heat resistant polyimide resin film is used as a material of the fixing belt and a Teflon(copyright) resin or silicone rubber layer adapted for releasing the toner is laminated on the resin film to prepare an endless belt.
On the other hand, JP ""679 discloses a fixing roll constructed as shown in FIG. 9. Specifically, it is taught that an aluminum mandrel having an outer diameter of 46 mm is covered with an intermediate HTV silicone rubber layer having a hardness of 45 Shore A and a thickness of 2 mm. Further, a silicone RTV rubber layer as a top coating layer having a thickness of 50 xcexcm is formed by a dip coating method on the surface of the intermediate layer to form an elastic rubber layer having a mirror surface. The endless belt transferred between the fixing roll and the pressurizing roll is formed of a polyimide film having, for example, a thickness of 75 xcexcm, a width of 300 mm and a circumferential length of 288 mm. Since the endless belt is formed of a hard polyimide film, it is considered reasonable to understand that the surface of the fixing roll is covered with the elastic rubber layer so as to impart elasticity to the fixing roll surface that is brought into contact with the toner.
Japanese Patent No. 2650864 proposes a fuser belt (fixing belt) of a three-layer structure as a seamless belt. It is taught that the belt comprises an inner resistance layer, an intermediate polyimide layer and an outer layer adapted for releasing the toner and made of a material having a low surface energy such as biton, Teflon(copyright) resin or silicone.
Further, Japanese Patent No. 2657990 proposes a fixing system in which a transfer material and a PET (polyethylene terephthalate) film having a thickness of 6 xcexcm are bonded to a pressurizing roll. In this prior art, the pressurizing roll is covered with a silicone rubber elastic layer.
The PET film used as a transfer-fixing belt is seriously defective in that the film is greatly shrunk thermally. The melting and fixing of the toner are performed in a fixing device heated to 220 to 250xc2x0 C. The thermal shrinkage of the PET film at 200xc2x0 C. reaches 5% in the vertical direction. Since the transfer-fixing belt is used at temperatures around the melting point (230xc2x0 C.) of PET, irregularities are generated over the entire surface of the transfer-fixing film, resulting in failure to perform a satisfactory fixation. On the other hand, polyimide, which is another main material of the transfer-fixing belt, is free from the defects described above. However, polyimide is highly costly.
The transfer-fixing belt consisting of a plastic film alone is generally low in tear strength, with the result that, where the edge portion of the belt is scratched or cracked, the cracking grows easily to cause the entire belt to be torn. Further, the transfer-fixing belt consisting of a plastic film alone is generally low in flexibility in the thickness direction, resulting in failure to be deformed along the toner image formed as a projecting portion on a copying material of a paper sheet or an OHP film (film, which is harder than a paper sheet, for an overhead projector). Therefore, when it comes to a picture image in which thick toner film portions and thin toner film portions are mixed at random, particularly a color picture image, a sufficiently high pressure is not applied to the thin toner film portions. It follows that a uniform fixation cannot be achieved.
Under the circumstances, it is very difficult nowadays to put a transfer-fixing belt made of a plastic film alone to a practical use.
An object of the present invention is to provide a transfer-fixing belt for an electronic appliance, the belt achieving a wide-nip feature, having a high heat conductivity, capable of improving the copying speed, rich in flexibility and excellent in dimensional stability, and the belt comprising a seamless reinforcing layer made of a continuous string, a heat resistant rubber layer formed on the reinforcing layer, and a heat resistant resin layer formed on the heat resistant rubber layer and adapted for releasing the toner attached thereto.
The transfer-fixing belt may further comprise a rubber paste layer on which the reinforcing layer is formed. Also, a continuous string covered with a rubber paste may be used for forming the reinforcing layer.
Another object of the present invention is to provide a method of manufacturing a transfer-fixing belt for an electronic appliance, the belt being capable of improving the copying speed, rich in flexibility and excellent in dimensional stability, and the method comprising the steps of forming a seamless reinforcing layer on a mandrel, forming a heat resistant rubber layer on the reinforcing layer, forming a heat resistant resin layer adapted for releasing the toner attached thereto on the heat resistant rubber layer, and withdrawing a laminate structure consisting of the reinforcing layer, the heat resistant rubber layer and the heat resistant resin layer from the mandrel.
According to a second aspect of the present invention, there is provided a method of manufacturing a transfer-fixing belt for an electronic appliance, compromising the steps of:
forming a rubber paste layer on a mandrel;
forming a seamless reinforcing layer on the rubber paste layer by continuously supplying a string to the mandrel while rotating the mandrel to have the mandrel wound with the string starting with one end portion of the mandrel;
forming a heat resistant rubber layer on the reinforcing layer;
forming a heat resistant resin layer adapted for releasing the toner attached thereto on the heat resistant rubber layer; and
withdrawing a laminate structure including the rubber paste layer, the reinforcing layer, the heat resistant rubber layer and the heat resistant resin layer from the mandrel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.